Recently, a PRAM (Phase-change Random Access Memory) which stores information by using a phase-change film attracts attention as a high-speed and long-life nonvolatile memory. The phase-change film is made of a material that changes its phase to an amorphous state having a high resistivity when it is heated at a high temperature (e.g., about 600° C. or above) and rapidly cooled, and to a crystalline state having a normal resistivity when it is heated at a low temperature (e.g., about 400° C. or above) and slowly cooled. The PRAM stores data by using the resistance difference between the two phases. The phase change is realized by controlling an amplitude of a current pulse. In other words, the phase is changed to the amorphous phase by applying a large current pulse and to the crystalline state by applying a small current pulse.
As for the material of the phase-change film used for the PRAM, Ge2Sb2Te5, a Ge—Sb—Te film, is used (e.g., Japanese Patent Application Publication No. 2008-103731). The Ge—Sb—Te film is generally formed by PVD such as sputtering or the like. Since, however, PVD yields insufficient step coverage, attempts have been made to apply CVD which provides good step coverage for film formation.
When the Ge—Sb—Te film is formed by CVD by using a Ge compound, a Sb compound and a Te compound as a film forming raw material, it has been found that smoothness of a film surface is deteriorated.